To receive and demodulate data of a wireless communication system, synchronization is required between a transmitter and a receiver. Especially, to successfully transmit and receive data in a wireless communication system in which a channel environment between a base station and a mobile station varies continuously, synchronization should be acquired through signaling between the base station and the mobile station. In downlink, since the base station simultaneously transmits a signal to a plurality of mobile stations in a reference timing, a synchronization problem does not occur. In uplink, however, since respective signals transmitted by multiple mobile stations arrive unequally, a propagation delay may be differently generated. Accordingly, an additional timing synchronization method is needed and one such method is ranging. Namely, ranging is a procedure for causing multiple mobile stations to correctly acquire synchronization for a transmission time.
A plurality of mobile stations acquires downlink synchronization using a data frame transmitted by a base station. As one method for the mobile stations to acquire synchronization, the base station may insert a preamble for synchronization in a part of a transmission frame. Therefore, the mobile stations acquire synchronization for a downlink channel through the preamble. Alternatively, the base station may use an additional synchronization channel.
In uplink, each mobile station should transmit data to the base station through a time zone and/or a frequency zone allocated thereto so that interference between mobile stations can be prevented and the base station can receive the data. Accordingly, synchronization acquisition through signaling between the base station and the mobile station in consideration of a channel environment of each mobile station is required for uplink synchronization.
Initial ranging refers to a process for obtaining accurate timing offset between a mobile station and a base station and initially adjusting transmission power. If a power source of the mobile station enters an ON state, the mobile station acquires downlink synchronization from a received downlink preamble signal. The mobile station then performs initial ranging to control an uplink timing offset and transmission power. Unlike the initial ranging, periodic ranging refers to a process for periodically tracking the uplink timing offset and received signal strength after the initial ranging. Bandwidth request ranging refers to a process for the mobile station to request that the base station allocate a bandwidth. Handover ranging is performed by the mobile station for synchronization with another base station during handover.
A wideband wireless communication system is based on orthogonal frequency division multiplexing (OFDM) scheme or orthogonal frequency division multiple access (OFDMA) scheme and can transmit data at a high rate by transmitting a physical channel signal using a plurality of subcarriers.
Standardization for a wireless access scheme of the wideband wireless communication system is conducted by the institute of electrical and electronics engineers (IEEE) 802.16 standardization group which is an international organization for standardization. The wireless communication system has evolved in a manner of improving a transmission rate compared to a conventional system by specification change or solving problems in terms of implementation of the conventional system.
A legacy system refers to a system conforming to a conventional specification and corresponds to a conventional system. For example, an IEEE 802.16e system corresponds to a legacy system. However, the legacy system is not limited only to the IEEE 802.16e system. A new system evolving from the conventional system may be installed in an area in which the legacy system is installed. In this case, the new system should be able to support a service to a new mobile terminal as well as a legacy mobile terminal.
A 16 m mobile station of an IEEE 802.16m system should perform a ranging process with a base station for initial access and bandwidth request. However, a ranging channel or ranging configuration information used when the 16 m mobile station performs the ranging process in a system supporting a legacy mode has not been specifically defined yet.